Battery charger

ABSTRACT

A charging circuit and method for charging a lithium-ion cell or battery at a charging voltage that is varied during the charging of the cell or battery from a selected minimum charging voltage to a predetermined maximum charging voltage. The charging circuit includes a transformer for transforming line voltage applied to the primary winding thereof to a lower AC secondary winding voltage, the transformer being selected to limit secondary winding output current when the charging voltage is not less than the selected minimum charging voltage to a value not exceeding a selected upper limit for the lithium-ion cell; a rectifier sub-circuit connected to the secondary winding of the transformer for rectifying the secondary winding voltage; and a charge-voltage regulator sub-circuit connected to the rectifier sub-circuit for receiving the rectified secondary winding voltage and providing an output charging voltage that is limited to the predetermined maximum charging voltage.

FIELD OF THE INVENTION

This invention relates to the field of devices and methods for charginglithium-ion cells (or batteries) and specifically to a charging circuitincluding a power transformer in which the loading curve of the powertransformer is used to limit the current flow to the lithium-ion cell(or battery) and a method for charging lithium-ion cells (or batteries)in which the loading curve of a power transformer is used to limit thecurrent flow to the lithium-ion cell (or battery).

BACKGROUND OF THE INVENTION

Lithium-ion cells are used in battery packs where high energy densityand low weight are required. However, lithium-ion cells can be dangerousif operated outside of their rated specifications. Typically, suchbatteries are used in controlled environments and are accompanied bysuitable protective devices to prevent such problems as short circuits,unduly high temperatures and over-discharge. A number of such protectivedevices are typically installed in the battery pack. It is standardindustry practice that lithium-ion cells are equipped with in-packcircuitry that provides the necessary protection for the cell in use.Although the in-pack circuitry will provide over-all protection,suitable cell charging circuitry is required to provide repeatedcharging of the cell while satisfying applicable charging andoperational constraints that vary somewhat from one cell type toanother, as the manufacturer may have specified for any given design.

Particularly, lithium-ion cells carry a risk of generating excess gasdue to overcharge or overdischarge—this may cause the safety vent of thebattery pack Lo open and release electrolyte into the atmosphere. Ifthis release of electrolyte is continued, the cells can lose sufficientelectrolyte that they are disabled. Further, overcharge or overdischargemay generate excess heat, causing a severe rise in temperature that canreduce the ability of the cell to retain energy and reduce the number ofcharging cycles the cell can undergo before it must be replaced. Moreseriously, overcharging or overdischarging may occur to such an extentthat the lithium metal is isolated from the other elements and maybecome plated onto one of the electrodes. Lithium metal is explosive inwater and will, in varying degrees, react with the moisture in theatmosphere. Lithium-containing batteries have been known to catch fire,although more recent safety designs have reduced the chances of thisoccurrence. The avoidance of overcharge voltage and overcharge currentduring charging of a lithium-ion cell is therefore an importantobjective in the use of lithium-ion cells, has been achieved by a numberof known regulator circuits, and is also a principal objective of thepresent invention.

It is known that the attained charge capacity of a lithium-ion cell issignificantly reduced if the charging voltage is less than themanufacturer's recommended maximum charging voltage (say 4.1 volts).With a drop of charging voltage of only 0.05V (approximately 1%), a lossof up to 5% in charge capacity occurs. However, if the charging voltagereaches only 4.0 volts (a drop of 0.1V or approximately 2%) then a lossof charge capacity of up to 12% occurs. On the other hand, as pointedout previously, if one exceeds the manufacturer's recommended maximumcharging voltage, the life cycle of the cell is decreased, or worse,catastrophic breakdown of the cell can occur. Therefore one is compelledby these combined constraints to charge the lithium-ion cell at avoltage (at least at the end of the charging cycle) that is as close asreasonably possible to the maximum charging voltage without exceedingit.

Previous battery charging circuits for lithium-ion cells or batteriesare known that include suitable regulator devices to maintain chargingvoltage and current within acceptable constraints. The “chargeinhibition voltage” refers to the value that the cell manufacturer hasset as the upper limit of operating/charging voltage of the cell. If thevoltage exceeds this value, lithium metal may become plated to anelectrode, with potentially dire consequences as discussed above. The“maximum charging voltage” is also established by the manufacturer at alower value than the charge inhibition voltage; if for example thecharge inhibition voltage is 4.35 volts for a representative cell, themaximum charging voltage is typically set at about 4.1 or 4.2 volts.Lithium-ion cell manufacturers have found that operation above themaximum charging voltage tends to reduce severely the recharging lifecycle of the battery. Accordingly, in order to ensure that chargingvoltage is no greater than the set maximum charging voltage for thecell, controlled lithium-ion cell charging circuits typically provide amaximum output charge voltage that is no more than the maximum chargingvoltage.

In a typical charging circuit, an alternating current source operatingat line voltage (typically 110-120 volts in North America) is applied tothe primary winding of a transformer whose secondary winding applies arelatively low AC voltage to a bridge rectifier. The output of thebridge rectifier is applied across a smoothing capacitor to the load(the load in the charging circuit is the lithium-ion cell or battery tobe charged). If no circuit elements were present other than theforegoing, the output voltage delivered to the lithium-ion cell would beat risk of exceeding the maximum charging voltage and ultimately mightexceed the charge inhibition voltage of the lithium-ion cell.Accordingly, interposed between the bridge circuit and the lithium-ioncell or battery is a regulator circuit for limiting the voltage andcurrent applied to the lithium-ion cell or battery during the chargingoperation.

A general purpose battery charger is described in U.S. Pat. No.3,736,490 (Fallon et al.). This patent describes a battery chargerincorporating a high leakage transformer and multiple rectifiers forregulating the charge current and the charge voltage applied to abattery. The high leakage transformer is used to provide impedanceisolation between the input and the output circuit of the transformerand thus to protect the semiconductor components from line transients.The transformer is selected for maintaining a trickle charge current tothe battery after a controlled rectifier providing supplemental currentto the battery has been cut-off.

Another charging device defining the general state of the art isdescribed in the abstract of Japanese Patent publication no 07296854(Mitsui). The abstract describes a device for charging a battery thatincludes a constant current generator for charging the battery at aconstant current at the initial stage of charging, and a constantvoltage generator for performing constant voltage charging after apredetermined charging voltage has been reached.

Two types of regulator circuit are conventionally used, both of whichare constant current/constant voltage regulator circuits, viz a linearregulator circuit, and a switching regulator circuit.

A switching regulator circuit includes a specially-designed chargecontrol integrated circuit (IC) device for use with the other circuitelements. Such IC device is connected within the switching regulatorcircuit in constant-current mode. With the regulator operating inconstant-current mode, charging continues at a constant current untilthe voltage across the lithium-ion cell or battery reaches the pre-setmaximum charging voltage. The circuit then limits the output chargingvoltage to the maximum charging voltage, using a pulse-width modulationtechnique. According to this technique, the length of time that chargecurrent is applied to the lithium-ion cell load during each AC. cycle isprogressively and gradually decreased as charging proceeds.

The commercially available Benchmarq™ model bq2054 IC device and the 4C™Technologies 4C-101656Li device are representative examples of chargecontrol IC elements for use with a switching regulator circuit of thetype described above.

As an alternative to the switching regulator, the principal otherpreviously known regulated lithium-ion cell charging circuit includes alinear regulator incorporating a pair of suitable linear regulatorcharge control IC devices, one such device being connected within acharge current regulation subcircuit of the overall charging circuit,and the other within a charge voltage regulation subcircuit. Theselinear IC devices incorporate transistors constrained to operate withina relatively linear region of operation which happens to be a relativelyinefficient region of operation. (By contrast, switching regulator ICdevices permit the transistors in the integrated circuit to operate inrelatively efficient Class C mode of operation.) Until fairly recently,such linear regulator circuits were considerably less efficient thanswitching regulator circuits, and generated an undesirable amount ofheat, although such linear regulators were typically lower in cost thanswitching regulators. For the older type of linear regulator, theminimum differential voltage (generally referred to as the “minimumdropout voltage”) between unregulated voltage at the input of the linearregulator circuit and the regulated output charge voltage of the linearregulator circuit was approximately 1.5 volts when used forconstant-voltage regulation and 1.2 volts when used for constant-currentregulation. As this differential voltage is relatively high, leading torelatively inefficient charging, linear regulators using the older typeof linear regulator IC device were typically used only for low-powercharging requirements.

A previously known battery-charging circuit not designed specificallyfor lithium-ion cells or batteries that uses only a single linearregulator charge control IC device that provides both charge currentregulation and voltage regulation is shown in FIG. 11-2 of GordonMcComb, Robot Builder's Bonanza (New York, 1987), p. 81. However, thatcircuit includes a current limiting resistor and a silicon-controlledrectifier and appears to be designed to provide constant chargingcurrent until the charging voltage reaches the maximum charging voltage.

More recently, a new generation of linear regulator charge control ICdevices has been developed that offers significant improvements inefficiency and a reduction in heat generation. These new regulators arefrequently referred to as low drop-out voltage regulators or “LDO”regulators, because the minimum differential voltage (dropout voltage)between input supply voltage and output charge voltage can be as low asabout 0.5 volts when used for constant-voltage regulation and as low as1.2 volts (about the same as for the older type of linear regulator ICdevice) when used for constant-current regulation of the chargingcircuit. The 0.5-volt differential when the IC device is operated inconstant-voltage mode permits these LDO regulators to operate from anunregulated DC supply voltage that is appreciably closer to the maximumcharging voltage than was the case for the older linear regulator ICdevices, thereby reducing power dissipation.

The older type of linear regulator charge control IC device isexemplified by the Motorola™ LM317 IC device. The more recentlyavailable LDO linear regulator charge control IC device is exemplifiedby the Micrel™ MIC29372 IC device.

Despite the improvements effected in IC design, lithium-ion cellcharging circuits of the types previously known remain inherentlyinefficient because they operate from unregulated DC power that issupplied at a voltage significantly above the maximum charging voltage;the inefficiencies are also due to the conventional use of bath chargecurrent and charge voltage regulating subcircuits, both of whichdissipate energy.

Prior art, such as European Patent document EP 0 825 699 A (BenchmarkMicroelectronics), teaches that both charge current and charge voltageshould be actively regulated during the charging of lithium-ion cell;the charge current initially at a constant value until the chargevoltage reaches the manufacturer's suggested maximum charging voltageand the charging voltage at the maximum charging voltage thereafter.Actively regulating current to a constant value requires that theregulating subcircuit be supplied with a high enough voltage that theregulator will not drop out of regulation as the charging voltageincreases to the maximum charging voltage

SUMMARY OF THE INVENTION

The conventional design approach heretofore taken for the design of theregulation of lithium-ion cell or battery charging circuits proceeds onthe premise that it is a good idea for the regulating circuit to beconstantly active and to be regulating charging voltage and/or chargingcurrent throughout the complete cell charging process. (Herein frequentreference will be made to the “cell” to be charged, it being understoodthat with appropriate adjustments, one may in each case charge a batteryof cells. Generally, a reference to a “battery” should be understood toinclude a reference to a single cell.)

According to the invention, the transformer used in the lithium-ioncharging circuit is selected so that its inherent current-limitingcharacteristic (loading effect) permits the circuit to charge thelithium-ion cell during an initial period in which the regulator circuitneed not perform any regulating function. This enables a satisfactoryregulator circuit to be designed according to the invention using only asingle charge control IC device that in an initial stage of the chargingoperation is in non-regulating mode, permitting the rectifiedtransformer secondary output to be applied to the lithium-ion cell withonly a minimum voltage drop across the single IC device (present in avoltage regulating subcircuit), as compared to two voltage drops acrosstwo IC devices (one in a current-regulating subcircuit and one in avoltage-regulating subcircuit) that would be present in conventionalcharging circuits, thereby affording substantial energy savings. Whenthe charge voltage reaches a pre-set threshold level, the regulatorcircuit functions for the remainder or the charging operation in amanner similar to that of previous voltage regulation subcircuits, butwith less overall power loss, since there is no separatecurrent-regulation subcircuit present.

Accordingly, the invention provides a charging circuit for a lithium-ioncell (or battery) including a selected suitable transformercharacterized by an inherent secondary output current-limitingcapability that meets the initial current-limiting needs of the chargingcircuit, in combination with a suitable rectifier circuit (that mayitself be of conventional design) and a linear charge-voltage regulatingsubcircuit that during the initial part of the charge cycle does notoperate in regulating mode. Otherwise the linear charge-voltageregulator subcircuit and the rest of the circuit may be of conventionaldesign, except that no separate charge-current regulator subcircuit isnecessary nor present, thereby avoiding the associated power dissipationthat occurs in such subcircuit present in conventional designs.

During the initial stage of the charging operation, charge voltage andcharge current are maintained within acceptable limits by the conditionof the discharged lithium-ion cell and the inherent secondary windingcurrent-limiting characteristic of the transformer itself, and thereforethe linear charge-voltage regulating subcircuit drops the supply voltageonly by a minimum voltage drop (the minimum dropout voltage) between therectified transformer secondary output and the lithium-ion cell beingcharged. The charge current applied during this initial stage slowlydeclines as the voltage across the cell being charged increases. Forthat reason, this initial mode of operation of the charging circuit maybe referred to as “taper current mode”, since the current tapers offfrom an initial value varying more or less linearly with time to areduced value.

During the later stage of the charging operation, the linearcharge-voltage regulating subcircuit operates in the same manner as aconventional such subcircuit to limit applied charge voltage to themaximum charging voltage, during which time charge current decreasessubstantially logarithmically in the same manner as would occur in aconventional charging circuit incorporating linear regulation.Preferably the linear regulator IC device used in the charge-voltageregulating subcircuit is of the LDO type for maximum efficiency andcharge capacity.

The inventor has found that the charge-current regulator subcircuit andthe consequent power dissipation associated with such subcircuit may beeliminated without significantly affecting the performance of thebattery charger while maintaining the charging voltage within safelimits. The elimination of the current-limiting subcircuit offers bothimproved energy efficiency and reduced cost of manufacture of thecharging circuit, because not only is one subcircuit eliminated, but therequired transformer can be smaller and lighter.

Note that it is important that the current rating and secondary voltageof the transformer be carefully selected, both to prevent damage to thecell during the initial charging stage and to provide an appropriatetransformer loading curve so that the supply voltage begins to beregulated after the desired portion of the charging cycle has beencompleted. Specifically, a current rating for the transformer should beselected that is not greater than the maximum charging current for thecell or battery suggested by the manufacturer. The secondary voltage ofthe transformer (and therefore the characteristics of the transformerloading curve) should then be selected so that when the maximum chargingcurrent is flowing through the secondary winding of the transformer, thevoltage supplied to the voltage regulating subcircuit is approximatelyequal to the sum of (1) a minimum charging voltage of the cell orbattery to be charged selected to be somewhat less than themanufacturer's nominal voltage rating of the battery and (2) the minimumvoltage drop across the voltage regulating subcircuit. To compensate forline voltage variations, it is advisable to select the secondary voltageof the transformer based upon the maximum expected transformer primaryvoltage, rather than upon the average primary voltage, to avoid havingthe current flow during the initial charging stage exceed thetransformer rating due to higher than average primary voltage.

A minimum charging voltage somewhat less than the nominal voltage isdesirable, although the exact voltage used is not critical. For example,the battery manufacturer's specifications for the battery for which thecharge is being designed should provide the charging voltage as afunction of time, assuming constant current until the charging voltagerises to the maximum charging voltage. In typical batteries known to theinventor, the charging voltage increases almost instantly from thedischarged voltage (which may be as low as 2.5 volts) to approximately3.3 to 3.7 volts reaching roughly 3.6 to 3.9 volts within a few minutes,depending upon a number of factors including the age and prior use ofthe battery. After the first few minutes the charging voltage continuesto climb, but somewhat more slowly, until it reaches the maximumcharging voltage of 4.1 or 4.2 volts as specified by the manufacturer(at which point the charging circuit must clamp the voltage or thebattery may be damaged). While an initial charging voltage of 3.4 voltsor even less could be used, the inventor has found that using an initialcharging voltage of 3.5 to 3.6 volts to select the current rating of thetransformer does not cause the charging current during the first fewminutes under charge to reach levels high enough to adversely affect thebattery being charged.

In accordance with the invention, for given battery specifications, thetransformer selected for the charger will have a lower power rating (alower current rating at the rated voltage) because the charging currentdecreases as the charging voltage increases. In a conventional chargerin which current is regulated to a constant value until the chargingvoltage rises to the maximum charging voltage, the power consumed by thecircuit must increase as the voltage rises as the current is being heldconstant. Hence the transformer must be rated to provide the maximumcharging current at the maximum charging voltage, rather than at theminimum charging voltage selected as discussed above. A transformer witha lower power rating is lighter, smaller, and less expensive andgenerates less heat.

As mentioned, in this specification, in many passages, reference will bemade to the charging of a lithium-ion cell; the representative voltagesand currents specified at various points in the charging circuit are fora representative such cell, and the charging circuit parameters for suchcell will be given typical values. However, it is to be understood thatthere is a variability in the characteristics ofcommercially-manufactured lithium-ion cells; such variability has to betaken into account in establishing various critical voltage and currentvalues within the charging circuit. Further, it is to be understood thata given charging circuit could be designed to charge two or more lithiumcells arranged in parallel or in series, and that depending upon theload for the circuit (i.e. the number of lithium-ion cells to be chargedand whether they are connected in parallel or series) such values againwould require adjustment from the typical values given in thisspecification.

The method according to the invention may be referred to as a “starvedregulator technique” or as a “tapered current/constant voltage”technique. Reference to a “starved regulator” is appropriate becauseduring the initial charging phase, the linear regulator IC device doesnot limit the charge voltage as the supply voltage is too low to requirelimiting. The regulator is starved for lack of voltage; this is not theway in which such regulators are designed to be used. The term “taperedcurrent/constant voltage” is appropriate because current steadilydiminishes as the threshold voltage is approached at which chargevoltage regulation commences; charge voltage is maintained at a constantvalue during the regulated stage of the charging operation.

While the invention is optimized if the more recently available LDOcharge-control IC device is used, the invention may also make use of theolder generation of linear IC devices, and in that event entailsadvantages of the sort recited in the preceding description relative topreviously known circuits that employ the older generation of linear ICdevices. In each case, the conventional current-regulating subcircuitcan be eliminated.

SUMMARY OF THE DRAWINGS

FIG. 1 is a circuit diagram of a charging circuit for a lithium-ion cellincorporating a linear regulator subcircuits of the type previouslyknown in the technology, and incorporating an older known type of chargecontrol IC device.

FIG. 2 is a circuit diagram of a charging circuit for a lithium-ion cellincorporating a linear regulator subcircuit of the type previously knownin the technology, and incorporating a more recent known type of chargecontrol IC device.

FIG. 3 is a circuit diagram of a charging circuit for a lithium-ion cellincluding a charge-voltage regulator subcircuit in accordance with theinvention, and incorporating an older known type of charge control ICdevice.

FIG. 4 is a circuit diagram of a charging circuit for a lithium-ion cellincluding a charge-voltage regulator subcircuit in accordance with theinvention, and incorporating a more recent known type of charge controlIC device.

FIG. 5 is a graph plotting the output voltage against output current ofa universal AC adaptor used in place of the transformer, bridgerectifier, and smoothing capacitor of FIG. 3 for the purpose of testingthe circuit shown in FIG. 3.

FIG. 6 is a graph plotting the voltage drop across and the currentthrough the lithium-ion cell of FIG. 1 during the operation of thecharging circuit of FIG. 1.

FIG. 7 is a graph plotting the voltage drop across and the currentthrough the lithium-ion cell of FIG. 3 during the operation of thecharging circuit of FIG. 3.

FIGS. 8-11 are graphs plotting the output voltage against output currentof AC adaptors rated at 300, 400, 800, and 1200 mA, respectively, eachused in place of the transformer, bridge rectifier, and smoothingcapacitor of FIG. 4 for the purpose of testing the circuit shown in FIG.4.

DETAILED DESCRIPTION WITH REFERENCE TO THE DRAWINGS

In the following, if a voltage is stated at a particular point in acircuit, it is to be understood that such voltage is measured relativeto ground. In each of FIGS. 1 through 4, the grounds are terminals G1,G2, G3, and G4, respectively.

FIG. 1 illustrates a conventional lithium-ion cell charging circuitwhose elements are interconnected in accordance with known technology.An alternating-current source 101, which may typically be a mains powersource at standard mains voltage (110-120 volts in North America),provides power to the input winding 103 of a transformer 105 whosesecondary winding 107 delivers an AC output that is rectified by abridge rectifier circuit 109 and is smoothed by smoothing capacitor 111.If desired, more elaborate smoothing may be provided in thisconventional circuit and in the charge circuit according to theinvention, to be described below. If the resulting unregulated DCcurrent applied at a voltage V_(W1) between terminals W1 and G1 in thecircuit were applied directly to lithium-ion cell 135 to be charged,there would be a serious risk of applying too high a charging current ortoo high a charging voltage, or both, to the lithium-ion cell 135,risking damage to the cell 135 and other hazards (including seriousinternal gas expansion within cell 135 and potentially an explosion).Accordingly, it is conventional to provide in such charging circuitregulator subcircuits to control the current and voltage applied to thecell 135.

If the lithium-ion cell 135 is nearly fully discharged to rated minimumdischarge voltage when it is connected to the circuit of FIG. 1 atterminals Z1 and G1 for recharging, there is no immediate risk orapplying too high a charging voltage (the fully discharged condition ofcell 135 precludes too high an initial charge voltage rise); theimmediate risk is that too high a charging current might be applied.Accordingly, the linear current regulator subcircuit comprising chargecontrol integrated circuit (IC) device 113 and resistor 119 ensures thatcharging current is kept within an acceptable range. Once the chargevoltage at terminal Z1 in the circuit reaches the maximum charge voltageacceptable for charging the lithium ion cell 135, a second regulatorsubcircuit limits the charge voltage to hold the charge voltage at orbelow a preset maximum voltage.

The second regulator subcircuit (charge-voltage subcircuit) comprisescharge control IC device 123, fixed resistor 129, and variable resistor133. Fixed resistor 129 and variable resistor 133 are used to set theregulated value of the charge voltage in accordance with theinstructions of the manufacturer of the charge control IC device 123.Resistors 129 and 133 may normally be omitted if the IC device 123 hasbeen designed by the manufacturer for the particular lithium-ion cell tobe charged.

The charge-voltage subcircuit limits the charge-voltage to the maximumvoltage for the particular lithium-ion cell to be charged.

The IC devices 113 and 123 are conventional and may each be one and thesame type of device, for example, the Motorola™ model LM317 chargecontrol IC device. The IC devices 113 and 123 are connected within theirrespective subcircuits in conventional manner. The input terminal 115 ofIC device 113 is connected to the positive output terminal W1 of thebridge rectifier 109. (Terminals W1, G1, Y1 and Z1 in the circuit may ormay not be correlatable with physically discrete terminals. as thecircuit designer may prefer). The output terminal 117 of IC device 113is connected to one terminal of resistor 119, and the other terminal ofresistor 119 is connected to the adjustment input terminal 121 of ICdevice 113. The bridge rectifier negative terminal G1 (which may beconsidered a ground line for the circuit) is connected to the negativeterminal of lithium-ion cell 135.

IC device 123 is similarly conventionally connected. The input terminal125 of IC device 123 is connected to terminal Y1, the output terminal127 of IC device 123 is connected at terminal Z1 to one terminal ofresistor 129 whose other terminal is connected to the adjustmentterminal 131 of IC device 123. The variable resistor 123 is connectedbetween the adjustment terminal 131 and terminal G1. If the IC device123 has been designed by the manufacturer as discussed above, thenterminal 131 (which would then be referred to as ground terminal 131) isconnected directly to the terminal G1; resistors 129 and 133 areomitted.

During the initial stage of the charging operation, the currentregulator subcircuit comprising IC device 113 and resistor 119 regulatescurrent, but IC device 123 provides an unregulated connection betweenits input terminal 125 and output terminal 127, since the output voltage(the charge voltage applied to lithium-ion cell 135) does not requireregulation during the initial stage of the charging operation. However,when the charge voltage at terminal Y1 reaches the established thresholdat which the maximum permitted charging voltage for application to cell135 appears at terminal Z1 (relative to ground voltage at terminal G1,of course), IC device 123 begins to regulate the output voltage atterminal Z1, maintaining it at the maximum permitted charge voltagevalue pre-set for charging the cell 135. From the time that IC device123 begins to regulate the output voltage at terminal Z1 that chargesthe cell 135, the charge current applied to cell 135 begins to declineapproximately logarithmically, and eventually approaches zero by thetime that the cell 135 is fully charged to the capacity permitted by thepre-set regulated charge voltage.

FIG. 2 is a charging circuit for a lithium-ion cell that is essentiallyidentical to the circuit of FIG. 1 except that charge control IC devices213 and 223 respectively have been substituted for charge control ICdevices 113 and 123 of FIG. 1. Otherwise, the circuit of FIG. 2 may beidentical to the circuit of FIG. 1. The charge control IC devices 213and 223 of FIG. 2 are the more recent “low drop-out voltage” or “LDO”type of IC devices capable of operating with a lower minimumdifferential voltage across charge control IC device 223. The IC devices213 and 223 are conventional and may each be one and the same type ofdevice, for example, the Micrel™ model MIC29372 charge control ICdevice.

FIG. 3 illustrates a charging circuit according to the invention for alithium-ion cell 335 that resembles, to a considerable extent, thecharging circuit of FIG. 1 but completely eliminates the charge currentregulator subcircuit of FIG. 1. In FIG. 3, an alternating current source301 provides power to the input winding 303 of a suitable transformer305 whose secondary winding 307 provides an output AC current that isrectified by bridge rectifier 309, the output of which is smoothed bysmoothing capacitor 311. The charging circuit illustrated in FIG. 3 willalso perform advantageously, at least for some applications, without thesmoothing capacitor 311, but the smoothing capacitor 311 is desirable toincrease the effective DC voltage and to correct the power factor.

The transformer 305 is selected not only for suitability to meet theusual charging circuit requirements, but also for its inherentcurrent-limiting capability during the initial mode of operation of thecircuit that enables the conventional current-limiting subcircuit to beeliminated. In the transformer 305, the windings ratio is selected toprovide an output AC voltage that after rectification delivers a DCsupply voltage across terminals W3, G3 that is sufficient to provide aregulated charge voltage at the preferred maximum pre-set value (whichDC supply voltage may be lower than that provided in the circuit of FIG.1 by at least the minimum dropout voltage of the charge control ICdevice 113), but with a voltage and current rating low enough to limitinitially the charge current in the manner discussed below. Otherwise,circuit elements 301, 305, 309, and 311 may be essentially identical tothe counterpart circuit elements 101, 105, 109, and 111 of FIG. 1 andare interconnected in essentially the same way.

However, in contradistinction to conventional charging circuits, theoutput of the bridge rectifier 309 applied across terminals W3 and G3 ofFIG. 3 is not regulated by any active current regulating device;instead, the circuit of FIG. 3 relies upon the inherent currentregulatory capability of the transformer 305 to limit charge current, aswill be further discussed below.

Charge control IC device 323 may be identical in type to IC device 123of FIG. 1, e.g. a Motorola™0 LM317 device, and is connected in thecircuit of FIG. 3 in generally the same way as IC device 123 isconnected in the circuit of FIG. 1. As is the case with FIG. 1, as longas the charge voltage applied across the lithium-ion cell 335 remainsbelow the designed maximum charge voltage for the circuit, chargecontrol IC device 323 does not regulate the charge voltage, but beginsto operate in regulation mode only when the charge voltage at terminalZ3 has reached the designed permitted maximum value. Accordingly, toachieve this charge voltage regulation, input terminal 325 of IC device323 is connected to the bridge rectifier output positive voltageterminal W3 (there being no intervening charge current regulatorcircuit), and the output terminal 327 of IC device 323 is connected tothe positive terminal of lithium-ion cell 335; the connection terminalis identified as Z3. Connected between the output terminal 327 and theadjustment terminal 331 of IC device 323 is the resistor 329 whoseresistance may be selected to be the same as that of resistor 129 ofFIG. 1, assuming that IC device 323 is of the same type as IC device 123of FIG. 1. Connected between the adjustment terminal 331 and “ground”terminal G3, which is connected to the negative terminal of lithium-ioncell 335, is an adjustable resistor 333 that can be essentiallyidentical to adjustable resistor 133 of FIG. 1, again assuming that ICdevice 323 is of the same type as IC device 123 of FIG. 1. Resistors 329and 333 may be omitted and terminal 331 (which would then be referred toas ground terminal 331) connected directly to “ground” terminal G3 ifthe IC device 323 has been designed by the manufacturer for theparticular lithium-ion cell to be charged.

FIG. 4 is a charging circuit for a lithium-ion cell 435 that isessentially identical to the charging circuit of FIG. 3 except that ICdevice 423 is of the “low drop-out voltage” or “LDO” type more recentlyavailable. Otherwise, the elements of FIG. 4 are essentially identicalto the counterpart elements of FIG. 3. Thus AC source 401, transformer405 having primary winding 403 and secondary winding 407, bridgerectifier 409 and smoothing capacitor 411 are essentially identical tothe counterpart elements 301, 305, 309 and 311 of FIG. 3, the output ofthe bridge rectifier 409 of FIG. 4 being applied across terminals W4 andG4. Terminal G4 serves as ground terminal for the circuit and isconnected to one terminal of adjustable resistor 433 and to the negativeterminal of lithium-ion cell 435. Resistor 429 may be of the sameresistance value as resistor 329 (again assuming identity of type of ICdevices 323, 423) and is connected along with adjustable resistor 433 tothe adjustment terminal of IC device 423 in essentially the same manneras resistor 329 and adjustable resistor 333 are connected in FIG. 3. Theinput, output and adjustment terminals of IC device 423 are identifiedby reference numerals 425, 427, and 431 respectively. The output voltageapplied at circuit terminal Z4 to the positive terminal of lithium-ioncell 435 is regulated by charge control IC device 423 in essentially thesame way as the voltage at terminal Z3 is regulated by the IC device 323of FIG. 3, the significant difference being that the minimum differencebetween the supply voltage at terminal W4 and the regulated chargevoltage at terminal Z4 is lower for the circuit of FIG. 4 than for thecircuit of FIG. 3 because the LDO charge control IC device 423 operatesat a lower dropout voltage.

IC device 423 may be essentially identical to the IC device 223 of FIG.2 and may be, for example, a Micrel™ model MIC 29372 device.

The reason that the charging circuits of FIGS. 3 and 4 are able tofunction successfully despite the absence of a charge-current regulatingsubcircuit is that the transformer 303 or 403, as the case may be, isselected so that its loading effect provides an inherentcurrent-limiting function. An understanding of this phenomenon isfacilitated by reference to the graph of FIG. 5.

FIG. 5 is a graph of the output voltage representative of that of atypical power transformer of the type that would be used in the circuitsshown in FIGS. 1-4 measured after rectification and smoothing. In FIG.3, this voltage would be measured at terminal W3. For convenience, thevoltage plotted in FIG. 5 is the measured output voltage of a 300mA-rated universal (multi-voltage) AC power adaptor set at the 6.0 voltsetting with a line voltage input of 110 volts AC and assuming avariable load over the range plotted, the variation in the loadcorrespondingly varying the current draw. The AC power adaptor includesa 300 mA-rated transformer corresponding to transformer 303 or 403 andrectification and smoothing components equivalent to those in FIGS. 1-4.Note that the 6.0 setting indicates an output voltage of 6.0 volts DC at300 mA with an input voltage of 120 volts AC rather than 110 volts AC.The lower input AC voltage used to obtain the data for FIG. 5 resultedin a reduction in the output voltage at 300 mA current to about 5.0volts DC.

As is apparent in FIG. 5, the rectified and smoothed DC voltage providedby an unregulated DC power supply (an example of which is the portion ofthe circuit shown in FIG. 3 between the alternating current source 301and terminal W3 of the smoothing capacitor 311) declines with increasingcurrent draw. To utilize this effect so that no charge-currentregulating subcircuit is needed, the battery charger designer mustselect an appropriate transformer. Rectification and smoothing may beaccomplished by a variety of known circuit designs. The use of designsfor rectification and smoothing other than that shown in FIGS. 1-4 mayresult in a different constant voltage drop across the rectifier fromthat discussed below, but the rate of decline of voltage with increasingcurrent will not be affected. To select a transformer appropriate foruse in a charging circuit for a particular lithium-ion cell or battery,the current rating and secondary voltage of the transformer must bedetermined. Cell/battery manufacturers generally suggest a charging rate(conventionally referred to as “C” or “C rate”) for a lithium-ion cellor battery of 0.5 C to 1.0 C to obtain an optimal compromise betweencharge time and cell or battery lifetime. The C rate is the value ofcurrent required to provide a given charge capacity within a given time,and its unit is defined so that a 1.0 C rate is a rate that dischargesthe cell or battery in 1 hour. For example, the 1.0 C rate for a 500 mAhbattery is 500 mA and a 0.5 C rate is 250 mA. A transformer currentrating should preferably be selected that is within the 0.5 C to 1.0 Crange, or at least not above the 1.0 C rate, to obtain optimal cell orbattery life and minimize transformer weight and size. (Lower C ratescan be chosen, but these appreciably increase the required chargingtime). The transformer's secondary voltage at the selected currentrating should then be selected so that the DC voltage supplied to thevoltage regulator (IC device 323 in the circuit shown in FIG. 3 andvoltage regulator IC device 423 in the circuit shown in FIG. 4) isapproximately equal to the sum of (1) an initial minimum chargingvoltage of the cell or battery to be charged (typically chosen asapproximately 3.5 or 3.6 volts based upon measurements of the chargingcharacteristics of the cell or battery to be charged) and (2) theminimum dropout voltage of the voltage regulator of approximately 1.8 to2.0 volts for a typical voltage regulator IC such as the LM317 (for lowdropout voltage regulators such as the MIC 29372 the minimum dropoutvoltage may be as low as approximately 0.8 volts, increasing inproportion to the load current). Typically the secondary voltage of thetransformer should therefore be selected so that the DC voltage suppliedto the voltage regulator is about 5.3 to 5.6 volts at the selectedcurrent rating for a voltage regulator IC such as the LM317. Selectionof a higher secondary voltage would cause current in excess of thetransformer's current rating to be drawn during initial charging, andselection of a lower secondary voltage would reduce the chargingcurrent.

To compensate for line voltage variations it is advisable to select thecurrent rating of the transformer as approximately the 1.0 C rate andthe secondary voltage of the transformer based upon the maximum expectedtransformer primary voltage to avoid the current exceeding thetransformer rating.

EXAMPLES Example 1

In a representative lithium-ion cell charging circuit in conformity withFIG. 3, the cell 335 (an NEC Moli Energy Corporation IMP220748) to becharged had a nominal 3.6 volt/500 mAh rating. A current rating of 300mA was selected, as it is a readily available current rating for powertransformers and is within the desired range for a 500 mAh cell, asdiscussed above. In testing the circuit, in place of the transformer305, bridge rectifier 309, and smoothing capacitor 311, a 300 mA rateduniversal (multi-voltage) AC power adaptor set at the 6.0 volt settingand supplied by an AC input voltage of approximately 110 volts was used(the same input voltage used to obtain data for the loading curveplotted in FIG. 5). Hence the unregulated voltage/current relationship,measured across the smoothing capacitor 311, is as shown in FIG. 5. AMotorola™ LM317 device was chosen as the IC device 323. Resistor 329 hada resistance of 2 kΩ and variable resistor 333 a maximum resistance of 1kΩ. In this example, the transformer 305 was selected based upon itscurrent rating at 110 volts AC input. As discussed above it ispreferable to use the current rating at the maximum expected linevoltage, which can be as high as 132 volts AC. However, as the currentrating selected was considerably less than the 1.0 C rate (300 mA ratherthan 500 mA), the transformer selected is appropriate. The examplesgiven below illustrate selection of transformer specifications basedupon maximum line voltage. While the current rating for transformerselected for this example at an input of 132 volts AC and a selectedvoltage of 6.0 volts is not known, it is expected that the rating wouldbe not be greater than 500 mA (maximum charging current suggested by themanufacturer).

As discussed above, charging occurs in two stages. In the first stage(before the voltage across the cell 335 reaches 4.1 volts), assume thatat a given time the cell 335 is partially charged so that the chargevoltage at terminal Z3 is, for example, 3.5 volts. The IC device 323 isset by the resistors 329 and 333 so that it will not regulate until thecharge voltage at terminal Z3 is 4.1 volts. As IC device 323 is notregulating voltage, its input voltage (terminal W3) will be higher thanthe voltage at its output (terminal Z3) of 3.5 volts by its minimumdropout voltage of approximately 1.8 volts, hence the voltage atterminal W3 will be approximately 5.3 volts. From the loadingrelationship shown in FIG. 5, the current drawn by the battery will belimited to approximately 300 mA, which is the rated current of thetransformer (in this example, the rated current of the power adaptor).

As the cell 335 becomes charged, the voltage at terminal Z3 willgradually increase until it reaches 4.1 volts and the second stage ofthe charging process begins. The voltage measured at terminal Z3 (thevoltage drop from terminal Z3 to terminal G3) is the sum of the batteryvoltage and the voltage drop across the internal resistance of thebattery due to the current flowing through the battery. Note that thebattery voltage is here distinguished from the voltage drop across thebattery measured at terminal Z3. The battery voltage will be slightlyless than 4.1 volts (or else charging would cease as no current wouldflow) and the continuing charging current will be decreasing as thebattery accepts further charge and the voltage increases toward 4.1volts. When the voltage at reaches 4.1 volts, the input voltage of theIC device 323 at terminal W3 will be approximately 5.6 volts. At 5.6volts, the transformer 305 will limit the current to approximately 200mA, as can be seen from FIG. 5. It can be seen that as the chargevoltage increased during the first stage, the charging current graduallydeclined, or “tapered down”, linearly from approximately 300 mA toapproximately 200 mA.

During the second stage, as the battery continues to charge, the batteryvoltage approaches 4.1 volts and the current through the battery mustdecline as the internal resistance is fixed and the voltage drop acrossthe internal resistance is the difference between the regulated voltageacross the battery and the battery voltage. The declining current causesthe voltage at terminal W3 to increase as the load on the transformer305 is further reduced, but the increased voltage at terminal W3 islimited by the IC device 323. This increases the voltage drop across ICdevice 323, but the current is declining rapidly so that the powerdissipated by IC device 323 decreases.

The behavior of the voltage measured at terminal Z3 (labelled “E”) andthe current (labelled “I”) passing through terminal Z3 during thecharging of cell 335 discussed above is illustrated by the chargingcurves shown in FIG. 7. For example, the transition from the first tothe second stage takes place at just under 4000 seconds.

Example 2

A similar illustration of the behavior of the voltage measured atterminal Z1 (labeled “E”) and the current (labelled “I”) passing throughterminal Z1 during the charging of cell 135 in the prior art circuitshown in FIG. 1 is shown in the charging curves of FIG. 6. Themeasurements used to plot FIG. 6 were obtained by using the same 300 mArated universal (multi-voltage) AC power adaptor that was used to obtaindata for the loading curve plotted in FIG. 5 except that the outputvoltage selector of the power adaptor was set at the 9.0 volt setting.The adaptor was supplied by an AC input voltage of 110 volts. In placeof the transformer 105, the bridge rectifier 109 and the smoothingcapacitor 111 shown in FIG. 1 the adaptor was used.

A comparison between the charging curves shown in FIG. 6 and those shownin FIG. 7 suggests that the circuit shown in FIG. 3, which is a batterycharger in accordance with the invention, is capable of charging alithium-ion cell in essentially the same time as the prior art chargercircuit shown in FIG. 1, but does so without the charge-currentregulating subcircuit of FIG. 1, provided that an appropriatetransformer current and voltage rating are selected.

Note that the universal (multi-voltage) AC power adaptor used in theexamples given above contains a multi-tap transformer and provides aselector switch for selecting a tap for the desired output voltage.(Neither a designer of a battery charger in accordance with the priorart nor a designer of a battery charger in accordance with the inventionwould be likely to use a multi-tap transformer except for testing, butwould instead select a power transformer with the desired current ratingand a fixed voltage rating. Nevertheless, the choice of such multi-taptransformer for testing purposes is not inappropriate.)

Example 3

As a further example, battery chargers for NEC Moli Energy Corporationlithium ion rechargeable batteries models IMP300648-1, IMP340848-1, andIMP341065 may be designed using single-voltage AC adaptors such modelsT35-4.4-300, T35-4.4-400, T35-4.4-800, and T35-4.4-1200 obtained fromENG Electric Co. Ltd., 3F No. 558, Hong Chang Twelve St., Taoyuan City,Taiwan ROC. Such AC adaptors contain a transformer, a bridge rectifier,and a smoothing capacitor so as to provided an unregulated DC powersupply for use as a battery substitute for battery powered devices.Since the adaptor inherently includes a transformer (305, 405), a bridgerectifier (309, 409), and a smoothing capacitor (311, 411), theseelements of FIGS. 3 and 4 need not be separately provided.

The specifications of NEC Moli Energy Corporation lithium ionrechargeable batteries models IMP300648-1, IMP340848-1, and IMP341065are provided in NEC documents Nos. PE2523 (Ver. 2), PE2526 (Ver. 3),PE2512 (Ver. 1), all published in April, 1999. The specifications ofearlier similar models are given in earlier publications. Each batteryis rated at a charge voltage of 4.2 volts and a nominal operatingvoltage of 3.8 volts.

Nominal capacities are 650 mAh, 1030 mAh, and 1650 mAh, respectively.For the purpose of designing a charger, the inventor has found that aminimum charging voltage somewhat less than the nominal operatingvoltage is desirable, although the exact voltage used is not critical.In this case, a minimum charging voltage of 3.6 volts is suggested bythe following considerations. The NFC documents mentioned above showplots of the charging voltage as a function of time. In each case, thecharging voltage increases almost instantly from the discharged voltage(which may be as low as 2.5 volts) to approximately 3.4 volts and climbswithin a short time on the order of minutes to roughly 3.8 volts. Fromthere it climbs somewhat more slowly until it reaches the maximumcharging voltage of 4.2 volts (at which point the charging circuit mustclamp the voltage or the battery may be damaged). While an initialcharging voltage of 3.4 volts could be used, the inventor has found thatusing an initial charging voltage of 3.6 volts to select the currentrating of the transformer does not cause the initial current to eachlevels high enough to adversely affect the battery being charged.Because initially the current and voltage are unregulated, if it happensthat a battery is charged that has been fully discharged and the currentrating of the transformer used in the charger was selected to be the 1.0C rate based upon 3.6 volts as an initial charging voltage, the initialcurrent will exceed the current rating of the transformer for a shortperiod. However, the inventor has found that the 1.0 C rate is notexceeded by a significant amount for long enough to cause harm to thebattery in such circumstances. Using a lower initial charging voltagefor selecting a transformer would mean that a smaller transformer with alower current rating would be chosen. Doing so would reduce the currentprovided to the battery throughout the charging cycle and thereforeadversely affect the charging rate. A compromise between selecting a lowinitial charging voltage, which would decrease the charging rate, and ahigh initial charging voltage such as 3.8 volts, which would increasethe possibility of damage to the battery in the initial portion of thecharging cycle (only at the maximum line voltage, of course), is to use3.5 or 3.6 volts as the initial charging voltage for the purposes ofdesigning the charger.

FIGS. 8, 9, 10, and 11 are graphs of the DC output voltage in volts as afunction of current in milliamps for adaptors T35-4.4-300, T35-4.4-400,T35-4.4-800, and T35-4.4-1200, respectively, based upon input voltagesof 132 volts AC. Input voltages of 132 volts should be used in selectingan AC adaptor or an equivalent transformer/bridge rectifier 307/309 or407/409 so that variations in line voltage with the normal range of 10%above the nominal line voltage of 120 volts AC will not cause thecurrent at the minimum charging voltage to exceed the maximum chargingcurrent of the battery. If the AC adaptor is selected so that themaximum charging current for a particular battery at the minimumcharging voltage is provided at an input voltage to the AC adaptor of132 volts AC, then lower (and therefore safer) maximum charging currentswill be provided at lower AC input voltages.

Example 4

Applying the inventive method discussed above, an AC adaptor for use ina battery charger within the scope of the invention for charging an NECMoli Energy Corporation lithium ion rechargeable battery modelIMP300648-1 (say) should be selected by finding an AC adaptor whichprovides a current of 650 mA or less at a voltage calculated as the sumof the 3.6 volt minimum charging voltage and dropout voltage of thelinear regulator that limits the voltage across the battery (e.g., ICdevice 423 in FIG. 4). Typically, the dropout voltage is approximately0.6 volts for a low dropout voltage device such as the Micrel™ model MIC29372. Hence an AC adaptor that provides 4.2 volts at a current of 650mA or less when provided with an input voltage of 132 VAC is optimal.

Example 5

Inspection of FIGS. 8-11 indicates that the T35-4.4-400 adaptor, whoseloading curve is plotted in FIG. 9, is an optimal choice for an NEC MoliEnergy Corporation lithium ion rechargeable battery model IMP300648-1,assuming that a low dropout voltage device such as the Micrel™ model MIC29372 is used as in the circuit shown in FIG. 4. The T35-4.4-800 andT35-4.4-1200 adaptors would not be usable as at 4.2 volts the currentprovided by each exceeds 650 mA. The T35-4.4-300 adaptor could be used,but would provide less current and therefore require more time torecharge the battery.

Similarly, of the AC adaptors under discussion, the best choices for theNEC Moli Energy Corporation lithium ion rechargeable batteries modelsIMP340848-1 and IMP341065 can be seen to be the T35-4.4-800 andT35-4.4-1200 adaptors, respectively. However, neither provides a full1.0C current and are hence not optimal.

Comment on the Examples:

It is convenient to construct prototype battery chargers in accordancewith the invention using such single-voltage AC adaptors such as the ENGElectric AC adaptors discussed above, as such adaptors are inexpensiveand readily available.

However, battery chargers in accordance with the invention may bemanufactured using discrete transformers 307, 407, bridge rectifiers309, 409, and capacitors 311, 411, as the case may be. As a furtheroption, a manufacturer of AC adaptors may simply modify its AC adaptordesign to add the IC device 323, 423 (and associated resistors 329, 429and 333, 433, if necessary), thereby producing a battery chargerconforming to FIG. 3 or FIG. 4 and falling within the scope of theinvention.

By contrast, the prior art of lithium-ion battery charger design teachesuse of a higher voltage transformers than those discussed above in thepresent set of examples, necessitating the use of a charge-currentregulating subcircuit, which in turn increases power dissipation lossesas illustrated below.

Power Consumption in the Examples:

The following discusses the typical power dissipation of the circuits ofFIGS. 3 and 4.

In taper current mode (IC device 323 not operating in voltage regulatingmode), the maximum power dissipation of IC device 323 is given by:

 P _(d)=(V _(d))(I _(out))=(1.5V)(0.3 A)=0.45 W

where

P_(d) is the power dissipated in the IC device 323;

V_(d) is the voltage drop across the IC device 323, i.e. the differencebetween the voltages at terminals Z3 and W3 in the circuit; and

I_(out) is the charge current supplied to the cell 335.

In constant-voltage mode (in which the IC device 323 is in regulatingmode), the maximum power dissipation of the IC device 323 is roughlygiven by:

P _(d)=(V _(W) −V _(Z))(I _(out))

where

V_(W) is a typical voltage at terminal W3 in the circuit during theconstant-voltage stage; and

V_(Z) is the voltage at terminal Z3 in the circuit.

Accordingly,

P _(d)=(6.4V−4.1V)(0.2 A)=0.46 W

The foregoing power dissipation losses at about 0.5 watt aresignificantly superior to power dissipation losses in the circuits ofFIGS. 1 and 2, in each of which, assuming similar circuit implementationbut necessarily involving a second charge control IC device in eachcircuit, power dissipation losses can easily exceed 1 watt.

If the circuit of FIG. 4 were substituted for that of FIG. 3 in theforegoing example, a further improvement in power dissipation losseswould result; such losses in a circuit essentially equivalent to thatdiscussed above but with a Micrel™ model MIC 29372 LDO device 423substituted for the Motorola™ LM317 device specified above are typicallyless than about 0.25 W.

Effect of AC Line Voltage Variations on Charging Time:

Testing of lithium ion batteries suggests that the first 80% of capacityof the battery is attained during the portion of charging before thecharging voltage reaches the maximum charging voltage specified by themanufacturer. Hence if 1000 mA of charging current is applied to a 1000mAh cell, the 80% capacity level would be attained in approximately 0.8hours.

The last 20% of capacity is attained during the constant voltage portionof charging. During that period, the battery determines the amount ofcurrent it can consume. The time the battery takes to attain the final20% of capacity is approximately 1 hour regardless of how much chargingcurrent is available. Therefore the total time it takes to charge a 1000mAh cell with a charging current of 1000 mA is approximately 1.8 hours.If only 500 mA of charging current is provided to a 1000 mAh cell, thefirst 80% of capacity would take 1.6 hours and the final 20% capacitywould be attained in again 1 hour. Therefore the total charging timewould be 2.6 hours.

If an AC adapter is selected for which the current of the AC adapter ata charging voltage of 3.6 volts is 450 mA with 108 volts AC input, 600mA with 120 volts AC input, and 850 mA with 132 volts AC input, then thecharging time to attain 80% capacity for 108 VAC input is 1000mAh×80%÷450 mA=1.78 hours, for 120 VAC input is 1000 mAh×80%÷600 mA=1.23hours, and for 132 VAC input is 1000 mAh×80%÷850 mA=0.94 hours. Sincethe last 20% will always take approximately 1 hour, the total times forthe various input voltages are 2.78 hours for 108 VAC, 2.23 hours for120 VAC, 1.94 hours for 132 VAC. For many applications this variation incharging times is not significant, especially in view of the reducedcost, size, and heat produced by a battery charge in accordance with theinvention.

The foregoing discussion has proceeded on the basis that the outputvoltage at terminals W3 and W4 in the circuits of FIGS. 3 and 4respectively is a DC voltage, but as a practical matter, there willcontinue to be some AC ripple in the voltage at this terminal in therespective circuit. However, the IC device 323 or 423 is effective tolimit the charge voltage applied to the lithium-ion cell 335, 435 tovalues that do not damage the cell.

While the foregoing circuits have been described in the context ofcharging a lithium-ion cell, it is apparent that the circuits haveutility whenever it is necessary to supply a charging current to abattery or the like, that during a first stage requires only that thecharging current be below a specified value, and during a second stageadditionally requires that the charging voltage be below a specifiedvalue.

Variations will occur to those skilled in the technology withoutinvolving any departure from the principles of the invention. Forexample, various other types of rectifier could be substituted for thebridge rectifier 309, 409, or various more elaborate smoothing circuitscould be substituted for the smoothing capacitor 311, 411.

The scope of the invention is not limited to the circuits illustratedand described but is as defined in the appended claims.

What is claimed is:
 1. A charging circuit for charging a lithium-ioncell or battery (335, 435) at a charging voltage that varies during thecharging of the cell or battery (335, 435) from a selected minimumcharging voltage to a predetermined maximum charging voltage,comprising: (a) a selected suitable transformer (305, 405) fortransforming AC line voltage applied to the primary winding (303, 403)thereof to a lower AC secondary winding voltage; (b) a rectifiersub-circuit (309, 409) connected to the secondary winding (307, 407) ofthe transformer (305, 405) for rectifying the secondary winding voltage;and (c) a charge-voltage regulator sub-circuit (323, 423) connectedbetween the rectifier sub-circuit (309, 409) and the lithium-ion cell orbattery (335, 435) for receiving the rectified secondary winding voltageand providing an output charging voltage that is limited to thepredetermined A maximum charging voltage; the charge-voltage regulatorsub-circuit (323, 423) being connectable to the lithium-ion cell orbattery (335, 435) for charging the lithium-ion cell or battery (335,435) by applying the output charging voltage across the lithium-ion cellor battery (335, 435); wherein (d) the charging circuit in operationprovides charging current to the cell or battery (335, 435) in twosuccessive stages, viz (i) a first stage, during which thecharge-voltage regulator sub-circuit (323, 423) operates innon-regulating mode thereby to apply output charging voltage across thelithium-ion cell or battery (335, 435) at a value below thepredetermined maximum charging voltage; and (ii) a second stage, duringwhich the charge-voltage regulator sub-circuit (323, 423) operates in avoltage-regulating mode thereby to apply output charging voltage acrossthe lithium-ion cell or battery (335, 435) at a value limited to thepredetermined maximum charging voltage; and wherein the charge-voltageregulator sub-circuit (323, 423) changes its mode of operation fromnon-voltage-regulating mode to voltage-regulating mode when the voltagecharging voltage across the lithium-ion cell or battery (335, 435)reaches the predetermined maximum charging voltage; furthercharacterized in that (e) the transformer (305, 405) is selected tolimit secondary winding output current when the charging voltage isgreater than the selected minimum charging voltage so that the secondarywinding output current will not exceed a selected upper limit for thelithium-ion cell (335, 435); (f) the charge-voltage regulatorsub-circuit (323, 423) is connected to the rectifier sub-circuit (309,409) for receiving the rectified secondary winding voltage; and (g)wherein when the charge-voltage regulator sub-circuit (323, 423)operates in non-voltage-regulating mode, the charge circuit providescharging current to the cell or battery which is limited by means of theloading effect of the transformer (305, 405).
 2. The charging circuit ofclaim 1, additionally comprising a smoothing sub-circuit (311, 411)connected between the rectifier sub-circuit (309, 409) and thecharge-voltage regulator sub-circuit (323, 423) for smoothing therectified secondary winding voltage supplied to the charge-voltageregulator sub-circuit (323, 423).
 3. The charging circuit of claim 2,characterized in that the transformer (305, 405) is selected on thebasis that the AC line voltage applied to the primary winding thereof isa predetermined maximum AC line voltage.
 4. The charging circuit ofclaim 1, characterized in that the transformer (305, 405) is selected onthe basis that the AC line voltage applied to the primary windingthereof is a predetermined maximum AC line voltage.
 5. The chargingcircuit of claim 1, wherein the transformer is selected on the basisthat the AC line voltage applied to the primary winding thereof is apredetermined maximum AC line voltage.
 6. The charging circuit of claim5, wherein the minimum charging voltage is selected to be approximatelyequal to the average of (1) the initial charging voltage of thelithium-ion cell or when the charging current is held at a constantlevel equal to the 1.0 C rate for the lithium-ion cell or battery and(2) the predetermined nominal voltage of the lithium-ion cell orbattery.
 7. The charging circuit of claim 5 for a single lithium-ioncell, wherein the minimum charging voltage is approximately 3.6 volts.8. The charging circuit of claim 7, wherein the upper current limit isselected to be not greater than the maximum rate for the lithium-ioncell or battery specified by the manufacturer of the lithium-ion cell orbattery.
 9. The charging circuit of claim 5, wherein the upper currentlimit is selected to be not less than the 0.5 C rate nor greater thanthe 1.0 C rate for the lithium-ion cell or battery.
 10. The chargingcircuit of claim 5, wherein the upper current limit is selected to beapproximately the 1.0 C rate for the lithium-ion cell or battery. 11.The charging circuit of claim 5, wherein the charge voltage regulatorsub-circuit comprises a selected suitable charge-control IC device whoseinput terminal is connected to the positive output terminal of therectifier sub-circuit, and whose output terminal is connected to thepositive terminal of the lithium-ion cell or battery to be charged, andwhose ground terminal is connected to the negative terminal of therectifier sub-circuit and to the negative terminal of the lithium-ioncell or battery to be charged.
 12. The charging circuit of claim 11,wherein the charge control IC device is of the low drop-out voltagetype.
 13. The charging circuit of claim 5, wherein the charge voltageregulator sub-circuit comprises a selected suitable charge-control ICdevice whose input terminal is connected to the positive output terminalof the rectifier sub-circuit, and whose output terminal is connected tothe positive terminal of the lithium-ion cell or battery to be charged,and whose adjustment terminal is connected to one terminal of anadjustable resistor whose other terminal is connected to the negativeterminal of the rectifier sub-circuit and to the negative terminal ofthe lithium-ion cell or battery to be charged, the adjustment terminalalso being connected to one terminal of a resistor whose other terminalis connected to the output terminal of the IC device.
 14. The chargingcircuit of claim 13, wherein the charge control IC device is of the lowdrop-out voltage type.
 15. A charging circuit for charging a lithium-ioncell or battery at a charging voltage that varies during the charging ofthe cell or battery from a selected minimum charging voltage to apredetermined maximum charging voltage, comprising: (a) a selectedsuitable transformer for transforming AC line voltage applied to theprimary winding thereof to a lower AC secondary winding voltage; (b) arectifier sub-circuit connected to the secondary winding of thetransformer for rectifying the secondary winding voltage; and (c) acharge-voltage regulator sub-circuit connected between the rectifiersub-circuit and the lithium-ion cell or battery for receiving therectified secondary winding voltage and providing an output chargingvoltage that is limited to the predetermined maximum charging voltage;the charge-voltage regulator sub-circuit being connectable to thelithium-ion cell or battery for charging the lithium-ion cell or batteryby applying the output charging voltage across the lithium-ion cell orbattery; wherein (d) the charging circuit in operation provides chargingcurrent to the cell or battery in two successive stages, viz: (i) afirst stage, during which the charge-voltage regulator sub-circuitoperates in non-regulating mode thereby to apply output charging voltageacross the lithium-ion cell or battery at a value below thepredetermined maximum charging voltage; and (ii) a second stage, duringwhich the charge-voltage regulator sub-circuit operates in avoltage-regulating mode thereby to apply output charging voltage acrossthe lithium-ion cell or battery at a value limited to the predeterminedmaximum charging voltage; (e) wherein the charge-voltage regulatorsub-circuit changes its mode of operation from non-voltage-regulatingmode to voltage-regulating mode when the voltage charging voltage acrossthe lithium-ion cell or battery reaches the predetermined maximumcharging voltage; (f) the transformer is selected to limit secondarywinding output current when the charging voltage is greater than theselected minimum charging voltage so that the secondary winding outputcurrent will not exceed a selected upper limit for the lithium-ion; (g)the charge-voltage regulator sub-circuit is connected to the rectifiersub-circuit for receiving the rectified secondary winding voltage; (h)when the charge-voltage regulator sub-circuit operates innon-voltage-regulating mode, the charge circuit provides chargingcurrent to the cell or battery that is limited by means of the loadingeffect of the transformer; and (i) the minimum charging voltage isselected to be less than the predetermined nominal voltage of thelithium-ion cell or battery and greater than the initial chargingvoltage of the lithium-ion cell or battery when the charging current isheld at a constant level equal to the 1.0 C rate for the lithium-ioncell or battery.
 16. The charging circuit of claim 15, additionallycomprising a smoothing sub-circuit connected between the rectifiersub-circuit and the charge-voltage regulator sub-circuit for smoothingthe rectified secondary winding voltage supplied to the charge-voltageregulator sub-circuit.
 17. The charging circuit of claim 16, wherein thesmoothing sub-circuit is a smoothing capacitor connected between thepositive and negative output terminals of the rectifier sub-circuit sothat the rectified secondary winding voltage supplied by the rectifiersub-circuit is applied across the smoothing capacitor.
 18. The chargingcircuit of claim 15, wherein the transformer is selected on the basisthat the AC line voltage applied to the primary winding thereof is apredetermined maximum AC line voltage.
 19. The charging circuit of claim18, wherein the minimum charging voltage is selected to be approximatelyequal to the average of (1) the initial charging voltage of thelithium-ion cell or when the charging current is held at a constantlevel equal to the 1.0 C rate for the lithium-ion cell or battery and(2) the predetermined nominal voltage of the lithium-ion cell orbattery.
 20. The charging circuit of claim 18 for a single lithium-ioncell, wherein the minimum charging voltage is approximately 3.6 volts.21. The charging circuit of claim 18, wherein the upper current limit isselected to be not greater than the maximum rate for the lithium-ioncell or battery specified by the manufacturer of the lithium-ion cell orbattery.
 22. The charging circuit of claim 18, wherein the upper currentlimit is selected to be not less than the 0.5 C rate nor greater thanthe 1.0 C rate for the lithium-ion cell or battery.
 23. The chargingcircuit of claim 18, wherein the upper current limit is selected to beapproximately the 1.0 C rate for the lithium-ion cell or battery. 24.The charging circuit of claim 18, wherein the connection between thenegative output terminal of the rectifier sub-circuit and the negativeterminal of the lithium-ion cell or battery to be charged is a directohmic connection, and the connection between the positive outputterminal of the rectifier sub-circuit and the input terminal of thecharge-control IC device is a direct ohmic connection.
 25. The chargingcircuit of claim 16, wherein the transformer is selected on the basisthat the AC line voltage applied to the primary winding thereof is apredetermined maximum AC line voltage.
 26. The charging circuit of claim15, wherein the charge voltage regulator sub-circuit comprises aselected suitable charge-control IC device whose input terminal isconnected to the positive output terminal of the rectifier sub-circuit,and whose output terminal is connected to the positive terminal of thelithium-ion cell or battery to be charged, and whose ground terminal isconnected to the negative terminal of the rectifier sub-circuit and tothe negative terminal of the lithium-ion cell or battery to be charged.27. The charging circuit of claim 26, wherein the charge control ICdevice is of the low drop-out voltage type.
 28. The charging circuit ofclaim 15, wherein the charge voltage regulator sub-circuit comprises aselected suitable charge-control IC device whose input terminal isconnected to the positive output terminal of the rectifier sub-circuit,and whose output terminal is connected to the positive terminal of thelithium-ion cell or battery to be charged, and whose adjustment terminalis connected to one terminal of an adjustable resistor whose otherterminal is connected to the negative terminal of the rectifiersub-circuit and to the negative terminal of the lithium-ion cell orbattery to be charged, the adjustment terminal also being connected toone terminal of a resistor whose other terminal is connected to theoutput terminal of the IC device.
 29. The charging circuit of claim 28,wherein the charge control IC device is of the low drop-out voltagetype.